Polyisocyanates are well known low temperature curing agents for active hydrogen containing polymers such as polymeric polyols. However, special equipment must be used when handling the polyisocyanates and more particularly when handling curable compositions containing the polyisocyanates. The handling problems can be dealt with by blocking the polyisocyanates with a blocking agent such as a low molecular weight alcohol to form urethanes. However, high curing temperatures must be used in the curing reaction. Polyisocyanates may also be dimerized to form uretdiones that are known to dissociate to free isocyanates on thermal curing. However, once again, high curing temperatures are required.